The realization that dinosaurs are closely related to birds raised the possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-bird dinosaur fossils were discovered with preserved feathers. Since then, about 30 genera of theropods have been discovered to be feathered.

Most of these fossils come from the Yixian formation in China. The fossil feathers of one specimen, Shuvuuia deserti, have tested positive for beta-keratin, the main protein in bird feathers, in immunological tests.[1]

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Sinosauropteryx fossil, first fossil of a definitively non-avialan dinosaur with feathers

After a century without clear evidence, well-preserved fossils of feathered dinosaurs were discovered during the 1990s, and more continue to be found. The fossils were preserved in a Lagerstätte — a sedimentary deposit exhibiting remarkable richness and completeness in its fossils — in Liaoning, China. The area had repeatedly been smothered in volcanic ash by eruptions in Inner Mongolia 124 million years ago, during the Lower Cretaceous. The fine-grained ash preserved the living organisms that it buried in fine detail. The area was teeming with life, with millions of leaves, angiosperms (the oldest known), insects, fish, frogs, salamanders, mammals, turtles, lizards and crocodilians discovered to date.

The most important discoveries at Liaoning have been a host of feathered dinosaur fossils, with a steady stream of new finds filling in the picture of the dinosaur-bird connection and adding more to theories of the evolutionary development of feathers and flight. Quill knobs were found an ulna of Velociraptor mongoliensis. These are associated with large, well-developed secondary feathers.[2]

Behavioural evidence, in the form of an oviraptorosaur on its nest, showed another link with birds. Its forearms were folded, like those of a bird.[3] Although no feathers were preserved, it is likely that these would have been present to keep eggs and juveniles warm.[4]

In 2011, samples of amber were discovered to contain preserved feathers from the Cretaceous period, with evidence that they were from both dinosaurs and birds. Initial analysis suggests that some of the feathers were used for insulation, and not flight.[5][6]

A number of non-avialan dinosaurs are now known to have been feathered. Direct evidence of feathers exists for the following genera, listed in the order currently accepted evidence was first published. The evidence consists of feather impressions, or convincing skeletal or chemical evidence. This would be the presence of quill knobs (the anchor points for wing feathers on the forelimb) or a pygostyle (the fused vertebrae at the tail tip which often supports large feathers).

For comparison: Archaeopteryx (1861; avialan)[7][8] definitely had working flight feathers with off-centre rachis (stem). Otherwise, apart from the front limbs, it was a typical small carnivorous dinosaur.

↑Gao, Cunling et al (2008) A new basal lineage Of early Cretaceous birds from China and its implications on the evolution of the avian tail. Palaeontology51, Part 4, pp. 775-791. doi:10.1111/j.1475-4983.2008.00793.x